Ethanol may be prepared by fermenting sugars such as molasses or by hydrating ethylene (hereinafter referred to sometimes as fermentation method and synthesis method, respectively).
Ethanol preparable by the fermentation method or by the synthesis method is an aqueous crude ethanol solution which is contaminated with various kinds of impurities. Major impurities contained in the aqueous crude ethanol solution prepared by the fermentation method include a variety of compounds, such as methanol, acetaldehyde, n-propanol, n-butanol, ethyl acetate, 3-methylbutanol, and so on. On the other hand, major impurities contained in the aqueous crude ethanol solution prepared by the synthesis method also include a variety of compounds, such as acetaldehyde, diethylether, acetone, sec.-butanol, n-butanol, crotonaldehyde, and so on.
As described hereinabove, the various kinds of impurities are contained in the aqueous crude ethanol solution, yet they are so minute in amount that the removal of such impurities is very difficult. In particular, alcohols having three or four carbon atoms (hereinafter referred to as C.sub.3 -C.sub.4 alcohols) are so similar in their physicochemical properties to ethanol that it is extremely difficult to remove them, as compared with the other impurities.
The distillation has generally been employed to purify an aqueous crude ethanol solution containing the various kinds of the sugars as described hereinabove. This distillation method requires a large number of distilling columns and it consumes a large quantity of steam for distillation. Further, at least one of the plurality of the distilling columns is operated by the extractive distillation method in such a manner that a crude ethanol once concentrated by distillation is added again to a large volume of water and the resulting mixture is fed to the extractive distilling column.
In addition, the C.sub.3 -C.sub.4 alcohols among the impurities can be removed by withdrawing a side stream from the side of the distilling column for concentrating the ethanol; however, a large amount of the ethanol is also withdrawn from the distilling column so that the withdrawn side stream is further distilled in vacuo to thereby recover the ethanol.
Hence, the conventional process for the purification of ethanol using a large number of distilling columns is very poor in efficiency of energy.
In order to improve the problem with the poor efficiency in energy as seen in the distillation method as described hereinabove, a process is proposed in Japanese Patent Laid-open Publication (kokai) Nos. 60-41,627 and 2-49,741, which involves removing lipophilic impurities contained in an aqueous crude ethanol solution by extraction with carbon dioxide in a liquidized state or with carbon dioxide gases in a supercritical state (hereinafter referred sometimes to as carbon dioxide extraction method).
By the term "lipophilic impurities" referred to herein is meant oxygen-containing compounds having two or more carbon atoms, except for ethanol. The aqueous crude ethanol solution preparable by the fermentation method contains methanol as a hydrophilic impurity, in addition to the lipophilic impurities.
The carbon dioxide extraction method can separate the lipophilic impurities as an extract and provide a mixed aqueous solution of a highly purified ethanol and methanol as a raffinate. The rate of extraction (removal) of the lipophilic impurities in this case increases in proportion to an increase in a ratio of an extractant (that is, a ratio of the weight of the extractant to the weight of the aqueous crude ethanol solution), and the predetermined lipophilic impurities can be extracted as the ratio of the extractant reaches to a somewhat large extent. The minimal ratio of the extractant may vary with the concentration of ethanol in the aqueous crude ethanol solution, the composition or the concentration of the impurities, the acceptable range of the impurities in purified ethanol, or the like. Further, the higher the ratio of the extractant, the larger the amount of the extractant to be employed; hence, a high pressure extraction column and a compressor should be made so large in scale and size that costs of equipment become expensive and costs of power required for the compression and the circulation of carbon dioxide gases become high.
It can be noted herein that the C.sub.3 -C.sub.4 alcohols are the lipophilic impurities contained in the aqueous crude ethanol solution, that are the most difficult to extract upon the extraction with carbon dioxide in the liquidized state or carbon dioxide gases in the supercritical state. In the conventional processes, a considerably large amount of the C.sub.3 -C.sub.4 alcohols are left unremoved in the raffinate although almost all amounts of the other lipophilic impurities can be extracted at a certain ratio of the extractant; hence, the ratio of the extractant should be increased to a higher extent in order to extract all the amounts of the lipophilic impurities.
The present invention has the object to provide a process for the purification of an aqueous crude ethanol solution, so adapted as to efficiently separate and remove C.sub.3 -C.sub.4 alcohols from the aqueous crude ethanol solution without causing any increase in the ratio of the extractant.